With the increased demand for magnetic recording medium such as audio recording tape or a video recording tape, higher standards of such media are being required. More specifically, there is a need to improve electromagnetic properties which make it possible for the media to reproduce an original sound and images, and for the media to have improved running ability and durability.
One factor effecting the above characteristics of magnetic recording tapes is the binder. The electromagnetic characteristics, running ability and durability of magnetic recording tapes prepared by coating a coating composition on a support and drying it are determined by the binder in which a ferromagnetic powder is homogeneously dispersed in a solvent.
Useful binders for the magnetic recording medium include vinyl chloride-vinyl acetate copolymer, which can contain, as a monomer, vinyl alcohol, acrylic acid, acrylate, maleic acid, maleate, etc.; a vinyl chloride-vinylidene chloride copolymer, a vinylidene chloride-acrylonitrile copolymer, nitrocellulose, polyurethane resin and epoxy resin.
A copolymer of vinyl chloride-vinyl acetatevinyl alcohol is the most often used binder. This binder is used due to the presence of an OH group in the vinyl alcohol component of the copolymer of vinyl chloride-vinyl acetate-vinyl alcohol which contributes to providing high dispersibility. It is well known that surface smoothness is improved when an amount of vinyl alcohol component is increased. However, it has been found that with a vinyl chloride-vinyl acetate-vinyl alcohol copolymer prepared by saponifying part of vinyl acetate of a vinyl chloride-vinyl acetate copolymer, the effect of surface smoothness is decreased when an amount of vinyl acetate component is increased. Additionally, electromagnetic properties of the magnetic recording tapes are deteriorated.
It is disclosed in Japanese Patent Application (OPI) Nos. 151067/80 and 7233/81 that a copolymer of which a vinyl acetate component is sufficiently saponified is used as a binder. In the former case the amount of vinyl alcohol component is not less than 8 wt %, and in the latter case the amount of vinyl alcohol component is 8 to 22 wt % and the amount of the unsaponified vinyl acetate component is 0.3 to 0.5 wt % as shown in examples.
It has been found that the dispersibility and surface smoothness of the magnetic recording tapes can be improved by decreasing the amount of the vinyl acetate component which is sufficiently saponified. However, as the result of further investigations it has also been found that the more the vinyl alcohol component is used, the worse orientation is and that the ratio of residual flux density to saturated flux density, that is, the squareness ratio is deteriorated. Accordingly, satisfactory output and input characteristic of the magnetic recording tapes could not be obtained. The reason is unclear but it is believed that a binder is so strongly adsorbed to the surface of the magnetic powder due to the presence of the OH group that the magnetic powder cannot move freely in the coating composition during orientation.
It has further been found that when the amount of the vinyl alcohol component is large, stains occur during tape running at high temperature and humidity and the running property is deteriorated.